Hydraulically Controllable Mechanical Seal

ABSTRACT

A controllable mechanical seal for sealing a shaft rotatable relative to a housing of a device which manipulates a fluid, the seal including: (i) a first face element having a first face surface, wherein the first face element is adapted to rotate with the shaft; (ii) a second face element having a second face surface, wherein the second face element is adapted to be supported within the housing; wherein the first face surface and the second face surface define a gap between the surfaces, physical dimensions of the gap contributing to defining a leakage rate of the fluid through the gap; wherein at least one of the first face element or the second face element includes at least one cavity wholly contained within the face element, the at least one cavity adapted to contain a hydraulic fluid; and wherein the at least one cavity is in fluid communication with at least one hydraulic intensifier and at least one pressure control valve, the at least one hydraulic intensifier being in pressure communication with a source of pressure; (iii) a sensor adapted to generate a signal indicative of the leakage rate; and (iv) a controller responsive to the signal for generating an output; wherein a state of the at least one pressure control valve is adapted to change in response to the controller output in order to increase or decrease the pressure of the hydraulic fluid in the at least one cavity, thereby deforming one of the first face surface or the second face surface to adjust the leakage rate.

This application is a continuation application of U.S. Ser. No.14/774,279 filed on Sep. 10, 2015, which is a national stage applicationof International Application No. PCT/US2014/028595 filed Mar. 14, 2014,which claims priority from U.S. Provisional Patent Application No.61/781,361 filed Mar. 14, 2013.

Provided is a controllable mechanical seal for sealing a rotatable shaftin the housing of a device which manipulates a fluid, such as a pump,against leakage of fluids along the shaft.

Mechanical seals may be formed with two face elements. One element maybe attached to the housing of the machine to be sealed, and the othermay be attached to and rotate with the shaft. One of the elements may befixed so that no movement of the element occurs axially relative to theshaft. This element is referred to as the fixed face element. The othermay be movable axially along the shaft, and is referred to as thefloating face element. The face elements are located in opposedrelationship to each other, and are arranged so that in response toopening force, closing force, or both, a sealing relationship will beobtained between them to control or prevent leakage along the shaft.

It has been found that a successful mechanical seal may be obtained, notwhen the elements are in direct physical contact with each other, butrather when a thin lubricating fluid film is provided between theiropposing face surfaces during steady state operation. This fluid filmprevents or reduces wear due to direct mechanical contact of theelements, thereby avoiding the possibility of mechanical damage orfailure of the seal. However, the thickness of the fluid film must notbe too large, as this will cause excessive leakage.

Further, it has been found that the thickness of the fluid film may bedetermined by the geometry of the face surfaces. In particular, if thetwo face surfaces are perfectly flat and parallel, such that a uniformgap is formed between them, the floating element may move into physicalcontact with the fixed element. In order to maintain a finite filmthickness so as to prevent such a collapse, a sufficiently large openingforce, which tends to move the floating element away from the fixedelement, may be generated by fluid pressure within the gap. For this tohappen, the opposing surfaces should not be parallel, but ratherconverge radially in the direction from the high pressure side to thelow pressure side of the seal. If this convergence is increased, theopening force will increase and the film thickness will be increased.Similarly, if the convergence is decreased, the film thickness will bedecreased.

Generally, mechanical seals are manufactured with the face elementsdesigned such that their face surfaces have a predetermined convergenceduring steady state operation of the machine, taking into account theanticipated thermal and mechanical deformations of the elements. Thus areasonable film thickness is realized. It should be understood that thethickness of the film is relatively small, on the order of approximately1 to 5 μm. The deformations are equally small, on the order ofapproximately 0.5 to 2.5 μm. Heretofore, conventional mechanical sealshave been designed and built very carefully based upon all theanticipated deformations so as to produce an acceptable film thicknessat the condition of steady state operation. The film thickness wasdependent upon the seal design characteristics, such as type ofmaterial, configuration, etc., and operating conditions such astemperature, pressure, speed, load and fluid characteristics. Thus thefilm thickness could not be controlled once the seal had been placedinto service. Accordingly, conventional seals would experience facedamage and wear when a wide range of operating conditions, includingtransient conditions, were encountered.

It is therefore desirable to provide a controllable mechanical seal inwhich the thickness of a thin fluid film separating two seal faceelements may be controlled by external means. This allows the thicknessof the film to be varied in response to changes in the operatingconditions in order to maintain an optimum film thickness for a widerange of operating conditions.

Previous attempts have been made to provide a controllable mechanicalseal utilizing piezoelectric materials in at least one of the sealfaces. The piezoelectric materials expand and contract based on thevoltage passing through the materials. While these materials may be ableto provide suitable control characteristics to mechanical seals, therequirement for providing electricity to the material is not ideal, andin some cases undesirable. Piezoelectrically controllable mechanicalseals are disclosed in U.S. Pat. No. 4,643,437, which is incorporatedherein as if fully written out below.

What is needed is a controllable mechanical seal wherein the thicknessof a lubricating film separating seal face elements is controlled by anexternally developed force which is applied to at least one of the faceelements to cause deformation of its face surface, wherein theexternally developed force is not developed using piezoelectricmaterials. By controlling this deformation, the convergence of theopposing face surfaces is adjustable. The opening force is thuscontrollable such that an optimum film thickness may be obtained for awide range of operating conditions.

Embodiments of the present subject matter are disclosed with referenceto the accompanying drawings and are for illustrative purposes only. Thesubject matter is not limited in its application to the details ofconstruction or the arrangement of the components illustrated in thedrawings. Like reference numerals are used to indicate like components,unless otherwise indicated.

FIG. 1 is a schematic diagram of a portion of a centrifugal pumpcomprising a mechanical seal.

FIG. 2 is a schematic diagram of the forces acting on an exemplarymechanical seal.

FIG. 3 is an embodiments of a hydraulically controllable mechanical sealface element.

FIG. 4 is an embodiments of a hydraulically controllable mechanical sealface element.

FIG. 5 is an embodiments of a hydraulically controllable mechanical sealface element.

FIG. 6 is a schematic diagram of an illustrative hydraulic controlsystem.

FIG. 7 is a schematic diagram of an illustrative hydraulic controlsystem.

Provided is a controllable mechanical seal for sealing a shaft rotatablerelative to a housing of a device which manipulates a fluid, the sealcomprising: (i) a first face element having a first face surface,wherein the first face element is adapted to rotate with the shaft; (ii)a second face element having a second face surface, wherein the secondface element is adapted to be supported within the housing; wherein atleast one of the first face element or the second face element aremovable axially along an axis of the shaft; wherein the first facesurface and the second face surface define a gap between the surfaces,physical dimensions of the gap contributing to defining a leakage rateof the fluid through the gap; wherein at least one of the first faceelement or the second face element comprises at least one cavity whollycontained within the face element, the at least one cavity adapted tocontain a hydraulic fluid; and wherein the at least one cavity is influid communication with at least one pressure control valve andoptionally at least one hydraulic intensifier, the at least one cavitybeing in pressure communication with a source of pressure via the atleast one pressure control valve or optionally via the at least onehydraulic intensifier; (iii) a sensor adapted to generate a signalindicative of the leakage rate; and (iv) a controller responsive to thesignal for generating an output; wherein a state of the at least onepressure control valve is adapted to change in response to thecontroller output in order to increase or decrease the pressure of thehydraulic fluid in the at least one cavity, thereby deforming one of thefirst face surface or the second face surface to adjust the leakagerate.

Controllable mechanical seals as described in the previous paragraph aredisclosed in “Feasibility Study of a Controllable Mechanical Seal forReactor Coolant Pumps” by John Wilson Payne, published Apr. 3, 2013 byGeorgia Institute of Technology, which is incorporated herein as iffully written out below.

As used herein, the term “in fluid communication” means fluid may betransported directly or indirectly between the two components which arein fluid communication.

As used herein, the term “in pressure communication” means that pressuremay be transmitted directly or indirectly between the two componentswhich are in pressure communication.

As used herein, the term “hydraulic intensifier” means a device whichreceives an input pressure from a source of pressure and modulates theinput pressure to provide an output pressure which is greater than orless than the input pressure. Exemplary hydraulic intensifiers includedevices which transform hydraulic power at low pressure into a reducedvolume at higher pressure. A specific, non-limiting example of ahydraulic intensifier may be constructed by mechanically connecting twopistons, each working in a separate cylinder of a different diameter. Asthe pistons are mechanically linked, their force and stroke length arethe same. If the diameters are different, the hydraulic pressure in eachcylinder will vary inversely to the ratio of their areas, the smallerpiston giving rise to a higher pressure.

The sensor may be any sensor which is capable of determining, directlyor indirectly, the leakage rate. Non-limiting examples of suitablesensors are sensors capable of determining temperature, pressure, flowrate and/or gap thickness. For example, a sensor capable of determiningtemperature may indicate whether face contact is imminent, which wouldindicate that the leakage rate has decreased to an undesirable level,providing an indirect determination of the leakage rate.

The controller receives a signal from the sensor and determines whatoutput is desired, if any, to change the state of the at least onepressure control valve in order to increase or decrease the pressure ofthe hydraulic fluid in the at least one cavity, thereby deforming one ofthe first face surface or the second face surface to adjust the leakagerate.

In certain embodiments, the controllable mechanical may include that theat least one cavity is in fluid communication with at least one pressurecontrol valve and at least one hydraulic intensifier, the at least onecavity being in pressure communication with a source of pressure via theat least one hydraulic intensifier.

In certain embodiments, either or both of the first face element and thesecond face element may comprise a metal, a ceramic material, or acarbon-based material. In certain embodiments, at least one of the firstface surface or the second face surface may comprise a ceramic materialor a carbon-based material coated onto at least one of the first faceelement or the second face element. In certain embodiments, the metalmay comprise steel. In certain embodiments, the ceramic material maycomprise at least one of aluminum oxide, silicon carbide or tungstencarbide. In certain embodiments, the carbon-based material may compriseat least one of graphite, resin-bound carbon or metal-bound carbon. Incertain embodiments, the steel may have an elastic modulus of about 200GPa and a Poisson ratio of about 0.3 and/or the graphite may have anelastic modulus of about 27 GPa and a Poisson ratio of about 0.3. By“metal”, what is meant is a metal or a metal alloy.

In certain embodiments, the controllable mechanical seal may comprise aplurality of cavities, each of the cavities being in fluid communicationwith a hydraulic intensifier via a pressure control valve for each ofthe cavities.

In certain embodiments, the controllable mechanical seal may comprise aplurality of cavities, each of the cavities being in fluid communicationwith a pressure control valve via a hydraulic intensifier for each ofthe cavities.

In certain embodiments, the hydraulic fluid in each of the cavities mayprovide a unique pressure in each of the cavities.

In certain embodiments, the controllable mechanical seal may comprisethree cavities.

In certain embodiments, the device which manipulates a fluid maycomprise a pump. The reactor coolant pump may be a centrifugal pump. Incertain embodiments, the source of pressure is a high-pressure side ofthe pump.

In certain embodiments, the device which manipulates a fluid maycomprise a reactor coolant pump associated with a reactor coolant systemof a nuclear reactor. The reactor coolant pump may be a centrifugalpump. In certain embodiments, the source of pressure is at least one ofthe reactor coolant system, a high-pressure side of the reactor coolantpump, or pressure provided by another pump within the reactor coolantsystem.

Nuclear power plants rely on cooling systems to ensure safe, continuousoperation of the nuclear reactor. Because of the large amount of heatgenerated by the fission reaction, the cooling systems demand a largevolumetric flow of water to maintain a safe operating temperature. Thecooling water may be supplied by one or more large centrifugal pumps. Inorder to maintain pump pressure and restrict water volume loss, thepumps typically utilize a multi-stage mechanical face seal system. Theseseals must operate with large pressure changes, potentially harsh waterchemistry, and possible high temperature excursions during their servicelife. As such, the seals used in nuclear reactor coolant pumps (RCPs)must be very robust.

Mechanical seals may be chosen for RCP sealing needs because of theirself-adjusting properties. These seals are designed to adjustautomatically to varying fluid conditions to provide the requiredsealing behavior. Over the service life of an RCP seal, it must operatecontinuously within a specified range of leakage rates. The designedleakage rate serves to lubricate the gap between the seal faces, or theface gap, while minimizing overall fluid loss. This lubrication preventsthe seal faces from coming into contact, which contact can causeaccelerated wear and damage of the seal faces, jeopardizing sealingintegrity. The lubrication also serves to cool the sealing components.The leakage rate of a mechanical seal is dependent on a variety offactors, including seal geometry and operating conditions. Two of themost important characteristics of a seal system are the face gap, or theaverage distance between the seal faces, and the coning, or the taper ofthe face gap from the inner diameter of the seal ring to the outerdiameter. In addition, the face gap and the coning are dependent on oneanother.

Nuclear power stations can experience difficulties over the service lifeof a seal due to a number of factors. Due to continuous operation andlong service lives, seal systems can experience gradual deviation fromnormal leakage rates. These deviations may be caused by an altered facegap. Over time, wear or chemical deposition may alter the face geometryof the seals, changing the behavior of the lubricating film and alteringthe face gap to produce too little or too much fluid leakage. If theleakage rate cannot be returned to an acceptable range, the nuclearreactor may be required to be shutdown, and replacement of the sealfaces may be necessary, which is extremely costly to the plant operator.Therefore, it may be desirable to extend both the service life of sealsystems and the ability of those systems to adjust to changing reactorcoolant system conditions.

FIG. 1 illustrates an embodiment of a mechanical seal 10 used with acentrifugal pump. The mechanical seal 10 includes a first face element12 which is fixed to and rotates with the shaft 14, and a second faceelement 16 which is fixed to the housing 18 and does not rotate.

As shown in FIG. 1, the first face element 12 is flexibly mounted to theshaft 14, such as by a spring 20, such that the first face element 12may travel along the shaft axis 22, and the second face element 16 isaxially fixed. However, it is also possible for the first element 12 tobe axially fixed and the second element 16 to be axially movable withthe shaft 14. It is also possible that both the first element 12 and thesecond element 16 are axially fixed, and that the biasing force requiredto close the mechanical seal 10 while the centrifugal pump is notoperating is provided by the weight of the seal itself. The first faceelement 12 and the second face element 16 restrict leakage by operatingin close proximity to one another, such that any leakage through themechanical seal 10 assembly must be through the sealing interface 26,also known as the face gap. Secondary seals 24 may help restrict fluidflow to the sealing interface 26. The spring 20 acts to close the facegap when the system is not rotating and provides a component of theclosing force when the system is rotating. While the shaft 14 isrotating, the sealing interface 26 is lubricated by a small amount offluid leakage through the sealing interface 26.

The sealing interface is a critical component of a mechanical seal. Theseal face elements move relative to one another and are in closeoperation, so careful design and operation is necessary to maintainoptimum sealing conditions in the sealing interface. In some seals, afull-film lubrication regime, in which the face elements do not contacteach other, is desired, and in other seals, mixed lubrication, withpartial face contact, is desired. For full-film lubrication, the sealinginterface must be greater than three times the root mean squareroughness of the seal faces; a smaller face gap will result in mixedlubrication. Full-film lubrication maximizes seal life by eliminatingwear caused by face contact during normal operation, but results in ahigher leakage rate. Mixed lubrication reduces the leakage rate, butwear and failures may occur more frequently due to sliding contact inthe faces.

The axial position of one seal face element relative to the other sealface elements determines the average fluid film thickness. This filmthickness influences all other behaviors in the face gap. Thesebehaviors include, but are not limited to, heat generation rate, fluidpressure, contact area, wear rate, and leakage rate. The axial positionof the seal face element which is movable along the axis of the shaft(the “floating face”) is determined by the forces acting on it; anequilibrium position is reached when the sum of axial forces is zero.Forces that act to close the face gap are closing forces and forces thatwiden the face gap are opening forces.

Referring to FIG. 2, the closing forces include a biasing force 40, suchas by a spring, and the pressure 42 exerted by the sealed fluid actingon the rear of the floating face 12. The force exerted by the pressureexerted by the sealed fluid may dominate this arrangement, in which casea biasing force 40 may not be required. For moderate to high pressures,the biasing force 40 may be negligible versus the pressure force 42. Theopening forces include the contact force 44 and the pressure 46 exertedby the sealed fluid acting on the face of the floating face 12.

As discussed above, at least one of the first face element or the secondface element may comprise at least one cavity wholly contained withinthe face element. FIG. 3 illustrates a face element 16 having a singlecavity 28. FIG. 4 illustrates a face element 16 having two cavities 28.FIG. 5 illustrates a face element having three cavities 28.

The face element(s) which include at least one cavity use hydraulicpressure either obtained from the discharge of the reactor coolant pump(RCP) (and thus limiting the maximum pressure to the full reactorcoolant system pressure, if a hydraulic intensifier is not used) or fromanother existing pump in the reactor coolant system (RCS).Alternatively, an additional pumping system may be used to pressurizethe hydraulic system for control. In certain embodiments, as shown inFIG. 5, three cavities are introduced into the seal face cross-section.These cavities may each be about 7 mm by about 8 mm in the dimensionsdepicted in FIG. 5, and may be evenly spaced within the seal face. Thecorners of the cavities may have a radius of about 0.5 mm to reducestress concentrations in the corners. Within these cavities, which willbe filled with hydraulic fluid during operation, a control pressure maybe applied which will induce a downward deflection of the seal face tocounteract the deflection caused by the sealed fluid pressure. Thisapplied (or control) pressure attempts to adjust the coning of the sealface, which will alter the leakage rate. While FIGS. 3 through 5 depictthe seals to be rectangular in cross-section, this need not be the case.In fact, it may be desirable to taper the face of the element 16 shownin FIGS. 3 through 5, as well as the opposing face, in order to providepre-coning.

Non-limiting illustrative examples of two control systems used tomoderate the hydraulic pressures in the face element depicted in FIG. 5are described below. Referring to

FIG. 6, the first example is a system 48 which uses a single point ofelectronic control to create three separate cavity pressures. Amicrocontroller 50 receives a signal from a sensor 62 indicating theleakage rate through the sealing interface 26. The microcontroller 50 isconnected to a single pressure control valve 52 which moderates thesource of pressure 54, such as the RCS pressure. Then, the controlledpressure 56 is fed into three independent hydraulic intensifiers 58. TheRCS side of each intensifier would be exposed to RCS water at thecontrolled pressure 56, and the seal face side of each intensifier wouldact on a sealed line 60 of hydraulic fluid, such as oil, which wouldindependently pressurize a single cavity 28. In certain embodiments, byselecting the pressure ratio of each hydraulic intensifier 58, threedifferent pressures proportional to one another may be achieved withinthe individual cavities 28. In the illustrative control method of FIG.6, the relationships between the three control pressures are fixed bythe selected hardware.

The second control system approach is shown in FIG. 7, which illustratesa system 49 which uses three independent electronically controlledvalves to provide the cavity pressures.

The system 49 of FIG. 7 is similar to that of FIG. 6, except that anoptional single hydraulic intensifier 58 is exposed to the source ofpressure 54 directly. The hydraulic intensifier 58 is not required inthe system 49 to adequately control the cavity pressures, but thehydraulic intensifier may be desirable in certain situations, such as ifit is desired to isolate the cavities 28 from the RCS water. Themicrocontroller 50 is connected to three independent pressure controlvalves 52 which moderate the pressure coming from the reactor coolantsystem, or optionally moderate the pressure of the hydraulic fluid, suchas oil, from the hydraulic intensifier 58. The method of FIG. 7 iscapable of employing software-based control of the cavity pressures andcan provide for any desired relationship (or none at all) between eachindividual cavity pressure.

It is noted that, although the present controllable mechanical seal hasbeen discussed with regard to use in centrifugal pumps used in nuclearreactor coolant systems, the controllable mechanical seal has uses inother industries and in apparatus other than pumps. The presentcontrollable mechanical seal is suitable for use in any apparatus whichrequires a seal around a rotatable body.

The following examples are set forth merely to further illustrate thepresent subject matter. The illustrative examples should not beconstrued as limiting the subject matter in any manner.

The following examples describe computer simulations which simulate theuse of hydraulically controllable or piezoelectrically controllablemechanical seals in centrifugal pumps for nuclear reactor coolantsystems. In all examples, the nominal leakage rate through themechanical seal is set to 11.36±0.11 L/min; this represents the leakagerate which is desired to be achieved by the controllable mechanicalseal. The closing force on the axially-movable seal face element isvaried in order to simulate variable operating conditions experienced bythe mechanical seal, and to determine the capabilities of the mechanicalseal to respond to varying operating conditions.

EXAMPLE 1

Example 1 is a hydraulically controllable mechanical seal including aface element made from 410 stainless steel, including three cavities asshown in FIG. 5. The pressure in the first cavity, near the outside ofthe radius of the mechanical seal, is P₁, the pressure in the second, ormiddle cavity is P₂, and the pressure in the third cavity, near theinside radius of the mechanical seal, is P₃. The value of P₁ isvariable, the value of P₂=P₁+2 MPa, and the value of P₃=P₁+4 MPa. Thenominal value of P_(i), or the value to which the system will defaultwhen not attempting to correct the leakage rate, is 5 MPa. This allowsthe mechanical seal to increase or decrease cavity pressure in order toincrease or decrease the deformation in the surface of the face element.Table 1 provides data which shows results achieved by the mechanicalseal at various closing forces.

TABLE 1 Closing P₁ Corrected Leakage Uncorrected Leakage Force (MN)(MPa) Rate (L/min) Rate (L/min) 0.490 12.11 11.39 14.24 0.492 9.46 11.4313.22 0.494 7.20 11.28 12.05 0.495 6.00 11.24 11.69 0.496 4.51 11.3211.21 0.498 2.26 11.26 10.25

Table 1 shows the value of P₁ which is required to obtain the correctedleakage rate at the specified closing force. The uncorrected leakagerate is the leakage rate at the specified closing force if P₁ defaultedto 5 MPa. Thus, the uncorrected leakage rate is the leakage rate whichthe mechanical seal is able to correct for at the specified closingforce. This shows that the mechanical seal including a face elementcomprising 410 stainless steel is able to correct for leakage ratesranging from 10.25 to 14.24 L/min.

EXAMPLE 2

Example 2 is a hydraulically controllable mechanical seal similar tothat of Example 1, except that the face element is made from carbongraphite available from SGL Group as EK-2200™ resin-bonded graphite. Thepressure in the first cavity, near the outside of the radius of themechanical seal, is P ₁, the pressure in the second, or middle cavity isP₂, and the pressure in the third cavity, near the inside radius of themechanical seal, is P₃. The value of P₁ is variable, the value ofP₂=P₁+2 MPa, and the value of P₃=P₁+4 MPa. The nominal value of P₁, orthe value to which the system will default when not attempting tocorrect the leakage rate, is 5 MPa. This allows the mechanical seal toincrease or decrease cavity pressure in order to increase or decreasethe deformation in the surface of the face element. Table 2 providesdata which shows results achieved by the mechanical seal at variousclosing forces.

TABLE 2 Closing P₁ Corrected Leakage Uncorrected Leakage Force (MN)(MPa) Rate (L/min) Rate (L/min) 0.460 11.67 11.36 25.61 0.465 10.0211.46 22.25 0.475 8.31 11.26 18.08 0.480 7.45 11.35 15.77 0.488 6.0111.46 13.11 0.490 5.60 11.41 12.47 0.492 5.40 11.27 11.98 0.494 5.2311.37 11.59 0.495 5.00 11.38 11.38 0.500 4.13 11.27 10.28 0.515 0.9711.28 7.44

Table 2 shows the value of P₁ which is required to obtain the correctedleakage rate at the specified closing force. The uncorrected leakagerate is the leakage rate at the specified closing force if P₁ defaultedto 5 MPa. Thus, the uncorrected leakage rate is the leakage rate whichthe mechanical seal is able to correct for at the specified closingforce. This shows that the mechanical seal including a face elementcomprising carbon graphite is able to correct for leakage rates rangingfrom 7.44 to 25.61 L/min.

Without wishing to be limited by theory, it is believed that the carbongraphite seal is able to correct for a greater range of leakage ratesbecause the carbon graphite is more elastic than the steel. It is notedthat the range of control is not perfectly centered around the nominalleakage rate. The seal is able to correct for a wider range of highleakage rates than low leakage rates. This behavior is desirable becauseplant operators more commonly experience high leakage rates than lowleakage rates. However, the performance of any of the proposedcontrollable seals can be tuned such that the range of leakage controlavailable is biased higher or lower by adjusting the pre-coning and/orthe closing force. This tuning can bias the range of control to addressthe most common instances of abnormal leakage rates, and could be usedin other applications where abnormal high leakage rates are not the mostcommon form of undesirable behavior.

EXAMPLE 3

Example 3 is a controllable mechanical seal including a face elementmade from piezoelectric material. The piezoelectric controllable sealuses an induced voltage across the face element to deform the surface ofthe face element. The structure of the simulations are similar to thosedescribed above with regard to Examples 1 and 2, except that the controlparameter is voltage across the face element, rather than cavitypressure. The nominal voltage, or the voltage to which the system willdefault when not attempting to correct the leakage rate, is 0 V. Thisallows the mechanical seal to increase or decrease voltage in order toincrease or decrease the deformation in the surface of the face element.Table 3 provides data which shows results achieved by the mechanicalseal at various closing forces.

TABLE 3 Closing Voltage Corrected Leakage Uncorrected Leakage Force (MN)(V) Rate (L/min) Rate (L/min) 0.490 4800 11.41 14.74 0.491 3800 11.3813.88 0.492 2900 11.41 13.25 0.493 1850 11.46 12.84 0.494 900 11.3812.09 0.495 0 11.43 11.43 0.496 −675 11.36 11.03 0.497 −1400 11.35 10.480.498 −2300 11.41 10.09 0.499 −3200 11.39 9.62 0.500 −4040 11.27 9.18

Table 3 shows the voltage which is required to obtain the correctedleakage rate at the specified closing force. The uncorrected leakagerate is the leakage rate at the specified closing force if the voltagedefaulted to 0 V. Thus, the uncorrected leakage rate is the leakage ratewhich the mechanical seal is able to correct for at the specifiedclosing force. This shows that the mechanical seal including a faceelement comprising piezoelectric material is able to correct for leakagerates ranging from 9.18 to 14.74 L/min. This range is slightly largerthan the range for the steel hydraulic seal, but significantly lowerthan the range for the carbon graphite hydraulic seal.

EXAMPLE 4

Example 4 is a controllable mechanical seal including a face elementmade from piezoelectric material, coated with a layer of graphite. Thepiezoelectric controllable seal uses an induced voltage across the faceelement to deform the surface of the face element. The structure of thesimulations are similar to those described above with regard to Examples1 and 2, except that the control parameter is voltage across the faceelement, rather than cavity pressure. The nominal voltage, or thevoltage to which the system will default when not attempting to correctthe leakage rate, is 0 V. This allows the mechanical seal to increase ordecrease voltage in order to increase or decrease the deformation in thesurface of the face element. Table 4 provides data which shows resultsachieved by the mechanical seal at various closing forces.

TABLE 4 Closing Voltage Corrected Leakage Uncorrected Leakage Force (MN)(V) Rate (L/min) Rate (L/min) 0.490 4300 11.41 14.02 0.491 3300 11.3813.51 0.492 2350 11.45 13.00 0.493 1625 11.38 12.37 0.494 800 11.3612.02 0.495 0 11.36 11.36 0.496 −800 11.37 11.10 0.497 −1500 11.32 10.550.498 −2350 11.31 10.11 0.499 −3125 11.27 9.59 0.500 −4020 11.46 9.10

Table 4 shows the voltage which is required to obtain the correctedleakage rate at the specified closing force. The uncorrected leakagerate is the leakage rate at the specified closing force if the voltagedefaulted to 0 V. Thus, the uncorrected leakage rate is the leakage ratewhich the mechanical seal is able to correct for at the specifiedclosing force. This shows that the mechanical seal including a faceelement comprising piezoelectric material with a graphite coating isable to correct for leakage rates ranging from 9.10 to 14.02 L/min.These results are very similar to those provided by Example 3.

It will be understood that the embodiments described herein are merelyexemplary, and that one skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as described hereinabove.Further, all embodiments disclosed are not necessarily in thealternative, as various embodiments of the invention may be combined toprovide the desired result.

What is claimed is:
 1. A controllable mechanical seal for sealing ashaft rotatable relative to a housing of a device which manipulates afluid, the seal comprising: i. a first face element having a first facesurface, wherein the first face element is adapted to rotate with theshaft; ii. a second face element having a second face surface, whereinthe second face element is adapted to be supported within the housing;wherein at least one of the first face element or the second faceelement are movable axially along an axis of the shaft; wherein thefirst face surface and the second face surface define a gap between thesurfaces, physical dimensions of the gap contributing to defining aleakage rate of the fluid through the gap; wherein at least one of thefirst face element or the second face element comprises at least onecavity wholly contained within the face element, the at least one cavityadapted to contain a hydraulic fluid; and wherein the at least onecavity is in fluid communication with at least one pressure controlvalve and optionally at least one hydraulic intensifier, the at leastone cavity being in pressure communication with a source of pressure viathe at least one pressure control valve or optionally via the at leastone hydraulic intensifier; iii. a sensor adapted to generate a signalindicative of the leakage rate; and iv. a controller responsive to thesignal for generating an output; wherein a state of the at least onepressure control valve is adapted to change in response to thecontroller output in order to increase or decrease the pressure of thehydraulic fluid in the at least one cavity, thereby deforming one of thefirst face surface or the second face surface to adjust the leakagerate.
 2. The controllable mechanical seal of claim 1, wherein at leastone of the first face element and the second face element comprise ametal, a ceramic material, or a carbon-based material.
 3. Thecontrollable mechanical seal of claim 1, wherein at least one of thefirst face surface or the second face surface comprises a ceramicmaterial or a carbon-based material coated onto at least one of thefirst face element or the second face element.
 4. The controllablemechanical seal of claim 2, wherein: (i) the metal material comprisessteel; (ii) the ceramic material comprises at least one of aluminumoxide, silicon carbide or tungsten carbide; and/or (iii) thecarbon-based material is at least one of graphite, resin-bound carbon ormetal-bound carbon.
 5. The controllable mechanical seal of claim 4,wherein the steel has an elastic modulus of about 200 GPa and a Poissonratio of about 0.3 and/or the graphite has an elastic modulus of about27 GPa and a Poisson ratio of about 0.3.
 6. The controllable mechanicalseal of claim 1, wherein the at least one cavity comprises threecavities.
 7. The controllable mechanical seal of claim 1, wherein the atleast one cavity is in fluid communication with at least one pressurecontrol valve and at least one hydraulic intensifier, the at least onecavity being in pressure communication with a source of pressure via theat least one hydraulic intensifier.
 8. The controllable mechanical sealof claim 2, wherein at least one of the first face surface or the secondface surface comprises a ceramic material or a carbon-based materialcoated onto at least one of the first face element or the second faceelement.
 9. The controllable mechanical seal of claim 3, wherein: (i)the metal material comprises steel; (ii) the ceramic material comprisesat least one of aluminum oxide, silicon carbide or tungsten carbide;and/or (iii) the carbon-based material is at least one of graphite,resin-bound carbon or metal-bound carbon.
 10. A device which manipulatesa fluid, comprising the controllable mechanical seal of claim
 1. 11. Thedevice of claim 10, wherein the device comprises a pump, optionally acentrifugal pump.
 12. The device of claim 11, wherein the source ofpressure is a high-pressure side of the pump.
 13. The device of claim10, wherein the device comprises a reactor coolant pump associated witha reactor coolant system of a nuclear reactor, wherein the reactorcoolant pump optionally is a centrifugal pump.
 14. The device of claim13, wherein the source of pressure is at least one of the reactorcoolant system, a high-pressure side of the reactor coolant pump, orpressure provided by another pump within the reactor coolant system.